Linear displacement or angular displacement is a basic physical value. Displacement sensors are extensively used in scientific research and industrial processes. Most of the displacement sensors being used today are potential type and photoelectric encoder type. Potential type displacement sensors are simple in structure. Being a traditional contact type sensor available at low cost, potential type displacement sensors' service life is short, and their accuracy and reliability are low. Photoelectric encoder type displacement sensor can measure angular displacement directly, and it can measure straight line displacement through a mechanical converter as well. However, its structure is complicated, and its production cost is high. Polarized light detection type displacement sensor can be designed with an external converter same with or similar to photoelectric encoder type displacement sensor, but its structure for the sensor portion is simple and its production cost is relatively low.
One of the basic issues needs to be solved for polarized light detection type displacement sensor is its inherent non-linearity, a revealed by Malus Law. The solution available today is by pre-calibration on mechanical components or by linearization through electronic circuit. However, pre-calibration would bring the need of a quite complicated mechanical structure, and the result of calibration would depend on mechanical precision. On the other hand, linearization of measurement data requires A/D, micro-controller and means the need of a complicated electronic circuit. Another basic issue needs to be solved for polarized light detection type displacement sensor is the drifting of light source intensity. The effect of the existing solutions, such as light feedback and temperature compensation, is very limited. The third issue needs to be solved for polarized light detection type displacement sensor is wide range measurement. The solution being used now is the utilization of the cyclic property of polarized light detection, which requires a continuous processing on the photoelectric signal, and means an increase of system complication and cost. For instance, the cost of absolute photoelectric encoder is much higher than incremental photoelectric encoder. The way to create a wide range absolute displacement sensor by using polarized light detection is an important issue needs to be solved.